In dual-fuel gas turbines, the turbine operates by burning either a gaseous fuel or a liquid fuel, the latter fuel typically being distillate oil. These gas turbines have fuel supply systems for both liquid and gas fuels. The gas turbines generally do not burn both gas and liquid fuels at the same time. Rather, when the gas turbine burns liquid fuel, the gas fuel supply is turned off, and when the gas turbine burns gaseous fuel, the liquid fuel supply is turned off.
In an exemplary industrial gas turbine, the combustor may have an array of combustion cans, each of which has a liquid fuel nozzle and a gas fuel nozzle. In the combustion can arrangement, combustion is initiated within the combustion cans at a point slightly downstream of the nozzles. Air from the compressor flows around and through the combustion cans to provide oxygen for combustion. Water injection nozzles are arranged within the combustor to introduce water to the combustion process for the purpose of reducing NOx emissions by reducing the peak flame temperatures.
During distillate operation, liquid fuel systems rely on flow dividers (non-driven gear pumps) to evenly distribute flow to each combustion can. Because gas fuel is used as the primary fuel, liquid fuel systems may remain inoperable for relatively long periods. If the flow dividers are not exercised regularly, they become vulnerable to having their gears bind. Regular exercise for a flow divider is conventionally accomplished during the weekly fuel transfers TIL 1107-3.
Despite the fact that customers are encouraged to exercise their liquid fuel systems at least once a week, this recommendation is not always heeded. In some instances, customers have valid causes for not following this recommendation. Customers reasons for not periodically running the liquid fuel systems may include reliability issues, emissions concerns and an unwillingness to decrease loads simply to transfer fuels, especially when power is trading favorably.
Existing F-Class gas turbines that have dual fuel capacity (gas fuel as primary and distillate as backup) are susceptible to carbon deposits forming in the liquid fuel system. Research indicates that carbon formation begins when distillate is heated to a temperature of 350xc2x0 F. in the absence of oxygen. In the presence of oxygen, the process accelerates and carbon formation begins at approximately 280xc2x0 F. As carbon deposits accumulate, they effectively reduce the cross-sectional passages through which the liquid fuel flows. If the carbon deposition continues, particles may clog the distillate passages. Since the carbon particles may not be present upstream of the turbine compartment, minimum passage sizes are not an issue until the distillate has been subjected to the turbine compartment""s heat.
When burning gas fuel the fuel nozzle liquid passages are purged but liquid fuel remains in the system up to the 3 way purge valve ready for an immediate fuel transfer. The gas fuel passages are purged when burning liquid fuel.
Differential pressures in the distillate and purge air lines serve to actuate three-way valves disposed between the flow dividers and the cans. When a turbine is operating on gas fuel, purge air, which runs at a higher pressure than the static liquid fuel system pressure during gas fuel operation, actuates the three-way valves such that distillate cannot enter any of the combustion cans. During liquid fuel operation, the fuel pump pressurizes the distillate so that its force is greater than that of the purge air. As a result, the piston within the three-way valve slides over to block the purge air flow and allow distillate into the combustion cans.
When a turbine is operating on gas fuel, as noted above, the liquid fuel system remains charged so that it is readily available for any fuel transfer requests. When liquid fuel systems remain inoperable beyond the recommended time limit, there is an increased likelihood that the static distillate within the turbine compartment will begin to experience carbon formation. Furthermore, due to the large difference in pressures, purge air often seeps across seals within the three-way valve""s internal cavities. Air then comes into intimate contact with distillate on the other actuating side of the three-way valve. As noted above, distillate carbon formation initiates at a much lower temperature in the presence of oxygen. Considering that F-Class turbine compartment temperatures have been measured in excess of 315xc2x0 F., carbon formation is even more likely to occur if the seeping purge air remains in contact with static distillate. As carbonaceous particles form, they pose the threat of clogging internal flow passages, which could result in a turbine trip while switching to liquid fuel operation.
Prior actions to prevent carbon formation include methods for dissipating heat from the turbine compartment, which have primarily focused on ventilating the surrounding air, and efforts to exercise the system to help prevent gear binding in flow dividers, as mentioned above.
FIG. 1 is a simplified schematic depicting the existing or conventional liquid fuel system. This particular schematic illustration is of the configuration associated with F-Class GE Gas Turbines. As illustrated, the liquid fuel system begins downstream of the fuel forwarding system. Thus, the liquid fuel flows into the current liquid fuel system configuration from the liquid fuel forwarding skid as illustrated at 10. During liquid fuel operation, fuel forwarding pumps provide distillate flow through the LP filters and to the inlet of the fuel pump 12. The fuel pump 12 creates positive distillate flow through the bypass control valve 16 and the stop valve 18. FIG. 1 corresponds to a turbine firing on natural gas with the distillate on stand-by. For that reason, the bypass control valve 16 and stop valve 18 are disposed to recirculate any distillate flow through respective bypass lines 20,22 to recirculation line 24. When the system is operating on liquid fuel, a portion is diverted to the flow divider 26 which evenly distributes flow to each combustion can 28, only one of which is illustrated in FIG. 1. Box 30 schematically illustrates the turbine compartment and the components that are contained within this compartment.
When a turbine is operating on gas fuel, as illustrated in FIG. 1, the liquid fuel system remains charged so that it is readily available for any fuel transfer request. But, system components sit idle while both control and stop valves 16,18 remain seated in their normally closed position. Purge air, which runs at a higher pressure than the static liquid fuel system pressure during gas fuel operation, actuates the three-way valve 32 associated with each combustor (only one of which is illustrated in FIG. 1) so that distillate cannot enter the respective combustion can 28. It is this same purge air, that actuates the three-way valve 32, that can seep past the seals in the three way valve, interact with distillate, and promote carbon formation.
The invention provides a recirculation system for circulating distillate during gas fuel operation so as to reduce or eliminate distillate carbon formation. Adding a recirculation system embodying the invention offers multiple benefits. First, it keeps the distillate""s temperature below the carbon formation limit by circulating the distillate back to a heat sink and/or heat exchanger. Second, the recirculating flow exercises the flow dividers"" gears without having to perform fuel transfers. Third, the system of the invention evacuates air from internal cavities around the three-way valves, which are the areas most likely to be exposed to air (oxygen) due to their operational nature.
The invention is thus intended to obsolesce the suggested practice that customers perform fuel transfers in order to exercise their liquid fuel systems. In addition to relaxing the recommendation for a periodic operation of the liquid fuel system, the recirculation system offers the benefit of increased reliability and availability.
The invention is thus embodied in a liquid fuel recirculation system for recirculating liquid fuel during gas fuel operation of a dual fuel gas-turbine, comprising: a valve for selectively directing liquid fuel to a liquid fuel nozzle of the turbine; a liquid fuel storage tank; at least one pump for pumping liquid fuel to said valve; a recirculation line for recirculating liquid fuel from said valve to one of said liquid fuel storage tank and said at least one pump; and a source of liquid fuel purge air operatively coupled to said valve; wherein said valve is constructed and arranged to shuttle between a liquid fuel mode wherein liquid fuel is directed to said liquid fuel nozzle, and a purge mode wherein liquid fuel is directed to said recirculation line and purge air from said purge air source is directed to said liquid fuel nozzle.
The invention may also be embodied in a system for recirculating liquid fuel during gas fuel operation of a dual fuel gas-turbine, comprising: a plurality of three-way valves, each for receiving liquid fuel from a liquid fuel flow divider and selectively directing the liquid fuel to a respective combustion can of the turbine; a liquid fuel storage tank; a primary liquid fuel pump for selectively pumping liquid fuel through said flow divider to said three-way valves; a recirculation pump for selectively pumping liquid fuel through said flow divider to said three-way valves; a plurality of recirculation lines, each for recirculating liquid fuel from a respective said three-way valve to a recirculating flow manifold; a common recirculating flow line for conducting liquid fuel from said manifold to at least of said pumps; and a source of liquid fuel purge air operatively coupled to each said three-way valve; wherein said three-way valves are constructed and arranged to shuttle between a liquid fuel mode wherein liquid fuel is directed to said combustion can, and a purge mode wherein liquid fuel is directed to said respective recirculation line and purge air from said purge air source is directed to said combustion can.
The invention also provides a method of reducing distillate carbon formation in a liquid fuel supply system during gas fuel operation of a dual fuel gas turbine comprising: providing a valve for selectively directing liquid fuel from at least one pump to a liquid fuel nozzle of the turbine; providing a recirculation line for recirculating liquid fuel from said valve to said at least one pump; communicating a source of liquid fuel purge air with said valve, wherein said valve is constructed and arranged to shuttle between a liquid fuel mode wherein liquid fuel is directed to said liquid fuel nozzle, and a purge mode wherein liquid fuel is directed to said recirculation line and purge air from said purge air source is directed to said liquid fuel nozzle; actuating said valve to said purge mode; operating a liquid fuel pump to direct liquid fuel to said valve; and recirculating said liquid fuel to said pump through said recirculation line.